WILEY ANTENNAS FOR PORTABLE DEVICES phần 4 pot

Figure 3.8 Simulated inductance of spiral coil antenna by IE3D.3.3.1.6 Case Study In this section, an example is presented to demonstrate the method for designing a coilantenna with a pr

RF circuit

Antenna coil

Control logic

Memory Microchip

Figure 3.6 (a) Typical inductive coupling tag and (b) its equivalent circuit

of the interaction between tags on the overall performance because the overall resonantfrequency of the two tags directly adjacent to one another is always lower than the resonantfrequency of a single tag [2] The resonant frequency of the parallel resonant circuit can becalculated using the Thomson’s equation:

The coil antenna is usually structured on a substrate, typically made of polyethyleneterephthalate (PET), polyvinyl chloride (PVC), or polyamide and consists of wound wire or

etched copper/aluminum strips Conductive polymeric thick film pastes can also be used forthe coil antenna by screen printing or dispensing for cost reduction Etched or screen printedcoil antennas are suitable for HF systems because low inductance is required There aremany types of inductors that can be used to realize the required inductance, and the spiralinductor is widely used in HF RFID systems.

A typical spiral inductor is illustrated in Figure 3.7 The conductive strip is wound eitherclockwise or counterclockwise This configuration ensures that the current in adjacent tracks

is in phase The resulting mutual inductance yields a significant increase in the spiralinductor’s self-inductance Connecting the microchip to the open ends of the spiral antennaforms a tag The microchip can be directly connected to the inner and outer ends of the spiralinductor It is more convenient to use an underpass to connect the end of the outer turn tothe centre for microchip assembly if more windings are required for a larger inductance.The inductance of the spiral inductor is determined by its area (L× D and the number ofwindings [15] The width of the tracks and the spacing between them are usually uniform,although they can be non-uniform The spiral inductor can be any closed loop such as

a square, rectangle, triangle, circle, semi-circle, or ellipse The square spiral inductor hasbeen widely used in practical applications because of its simple layout

No analytical formula can be used to calculate the inductance of such spiral inductors.The calculation must instead be done by numerical methods Many commercial softwaretools such as ADS, IE3D, and Microwave Studio can be used for this purpose Figure 3.8shows the calculated inductance of the spiral inductor shown in Figure 3.7(b) with length

L= 50 mm, width D = 40 mm, strip width W = 1 mm, strip spacing S = 0.5 mm, number ofwindings N = 6, polyester substrate, r= 4, and thickness h = 50 m The calculation wasdone using IE3D software, based on the method of moments [16] The inductance is about2.3 H from 10 MHz to 17 MHz

For tags operating in frequency range below 135 kHz, a chip capacitor (Cp≈ 20–220 pF)

is generally required to achieve resonance at the desired frequency At high frequencies(13.56 MHz, 27.125 MHz), the required capacitance is usually so low that it can be provided

by the internal capacitance of the microchip and the parasitic capacitance of the coil Ingeneral, the internal capacitance of the microchip is fixed, thus the inductance of the coilantenna has to be modified for circuit resonance by varying its geometry

3.3.1.5 Q Factor

To characterize the coil antenna, the quality factor Q is commonly used The Q factor is

a measure of the ability of a resonant circuit to retain its energy A high Q means that acircuit leaks very little energy, while a low Q means that the circuit dissipates a lot of energy

Connection points for microchip

D

L

(a)

Connection points for

L

(b)

Figure 3.7 Spiral antenna: (a) same layer connection; (b) using two metal layers with an underpass

In Figure 3.6, the entire tag circuit can be considered as a parallel RLC circuit, where Rrepresents the entire ohmic losses of the tag, including the ohmic loss of the coil antennaand the series resistance of the microchip In this case, Q can be defined as

Q=2fL

The induced voltage of the coil antenna is proportional to the Q factor Usually, the Qfactor is maximized for a long reading distance, but it has to be noted that a high Q factorlimits the bandwidth of transmitted data Therefore, the typical Q value for most tag coilantennas is about 30–80

Figure 3.8 Simulated inductance of spiral coil antenna by IE3D.

3.3.1.6 Case Study

In this section, an example is presented to demonstrate the method for designing a coilantenna with a prior selected microchip Important issues such as the essential procedures ofgetting necessary information from a microchip datasheet, determining required inductance,configuring and simulating the coil antenna, and calculating the Q factor will be addressed.The EM4006 microchip [17] is a CMOS integrated circuit used in electronic read-onlytransponders and operating at 13.56 MHz Generally, the characteristics of the microchipcan be found in the datasheet provided by the manufacturer The most important pieces

of information for coil antenna design are the internal capacitance of the chip and thepad position configuration of the microchip The electrical parameters and pad position ofEM4006 are shown in Table 3.3 and Figure 3.9, respectively Once we have the information,

we can carry out the coil antenna design

Calculating Required Inductance of Coil Antenna

The internal capacitance, CRES, shown in Table 3.3, is required in coil antenna design It

is found that the typical value of CRES is 94.5 pF at 13.56 MHz Using (3.3), the requiredinductance of the coil antenna is:

2× 314 × 1356 × 1062× 945 × 10−12 = 146 H (3.6)

Coil Antenna Configuration and Simulation

Having calculated the required inductance of the coil antenna, the next step is to configure

a coil antenna according to the specific design requirements such as size constraint and theproperties of the substrate used The shape of the coil can be square, rectangle, triangle,ellipse, or any other closed structures Figure 3.10 shows the coil antenna layout in IE3D withthe parameters: L= 47 mm, D = 47 mm, strip width W = 1 mm, strip spacing S = 0.5 mm.The substrate is polyester, 50 m thick ( = 4.0, tan = 0.002) With the input impedance

Table 3.3 Electrical characteristics of EM2006.

Voltage drop VREC= (VC1− VC2− VDD− VSS

VDD= 2 V VSS= 0 V fC1= 1356 MHz sine wave, VC1 = 10V pp centered at VDD− VSS/2

Ta= 25˚C, unless otherwise specified

Q= XA

RA+ RS

= 1240

Antenna Pad Configuration

Much attention should be devoted to the antenna pad position configuration when the coillayout is made The coil tracks at the input of the antenna (antenna pad) must be adequatelyconfigured to fit the pad position of the microchip for tag assembly A proper antennapad configuration enables the microchip to be easily affixed to the antenna, which reducesrejection rate and the cost of the tag

Referring to Figure 3.9, the microchip has two pads C1, C2which are fixed on the ends

of the coil antenna The distance between C1 and C2 is 0.74 mm The antenna pad should

be configured so that the microchip can be placed and aligned on it properly The details ofthe antenna pad are shown in Figure 3.11 The width of the strips is tapered from 1.0 mm to0.5 mm; the spacing changes from 1.0 mm to 0.2 mm This arrangement ensures the properaffixing of the microchip to the coil antenna as long as the outline of the microchip is keptwithin the area of the antenna pads

740

1144 1124

316 1800

Other pads size : 76 × 76

TOUT VDD

EM4006

Figure 3.9 Microchip pad information: (a) pad assignment; (b) pad position

Antenna Pad

Figure 3.11 Details of the antenna pad configuration

3.3.2 Far-field RFID Tag Antennas

For far-field RFID systems, the tag antenna design plays a vital role in system efficiency andreliability since the operation of passive RFID tags is based on the EM field they receivefrom the readers Figure 3.3 illustrates the operating principles of a passive far-field RFIDsystem The reader sends out a continuous wave RF signal containing alternating currentpower and clock signal to the tag at the carrier frequency at which the reader operates The

RF voltage induced on the antenna terminals is converted to direct current which powers up

the microchip A voltage of about 1.2 V is necessary to energize the microchip for readingpurposes For writing, the microchip usually needs to draw about 2.2 V from the reader’s signal.Then the microchip sends back the information by varying complex RF input impedance Theimpedance typically toggles between two different states (conjugate matched and some otherimpedance) to modulate the backscattering signal When receiving this modulated signal, thereader decodes the pattern and obtains the tag information.

Power Link (Reader to Tags)

Consider the RFID system shown in Figure 3.4, where the output power of the reader is

Preader-txthe gain of the reader antenna is Greader-antthe distance between the reader antennaand the tag is R, and the gain of the tag antenna is Gtag-ant According to the Friis free-spacetransmission formula, the power received by the tag antenna is [18]:

Ptag-ant=

 4R

2

matching coefficient between the reader antenna and tag antenna If the two antennas are

the reader antenna is circularly polarized while the tag antenna is linearly polarized, hence

The maximum reading distance for a radio power link is obtained when Ptag-chip is equal

to the threshold power of the microchip, Ptag-threshold, which is the minimum threshold power

to power up the microchip on the RFID tag:

= 276 − 20 logfMHz + Preader-txdBm+ Greader-antdBic

Backscatter Communication Link

The backscatter communication link from the tags to the reader is largely dependent on thebackscatter field strength of the tag Based on a monostatic (backscattering) radar equation[19], the amount of modulated power received by the reader is given by:

Preader-rx= 2

43R4Preader−txG2

where  is the radar cross-section (RCS) of the RFID tag

When the received power is equal to the reader’s sensitivity, Preader-threshold, the maximumdistance for backscatter communication link can be obtained:

Rbackscatter= 4

 2

of the tag, , the RCS of the tag, , the threshold power of the microchip, Ptag−threshold, andreceiver sensitivity of the reader, Preader−threshold The last two parameters are predeterminedfor a prior selected reader and microchip The remaining parameters can be optimized toachieve a longer reading distance The above-mentioned parameters will be addressed in thefollowing sections

3.3.2.2 EIRP and ERP

As mentioned in Section 3.3.2.1, the maximum reading distance is proportional to the outputpower of the reader and the gain of the reader antenna Higher output power and gain ofthe reader antenna can offer a longer reading distance However, the output power is alwayslimited by national licensing regulations

EIRP is the measure of the radiated power which an isotropic emitter (i.e G= 1 or 0 dB)will need to supply in order to generate a defined radiation power at the reception location

as at the device under test [2]:

In addition to the EIRP, ERP is frequently used in radio regulations and in the literature.The ERP relates to a dipole antenna rather than an isotropic emitter It expresses the radiatedpower which dipole antenna (i.e G= 164 or 2.15 dB) will need to supply in order to

generate a defined radiation power at the reception location as at the device under test.

It is easy to convert between the two parameters:

Table 3.2 summarizes regulated EIRP or ERP in the UHF band for different countries/regions

3.3.2.3 Tag Antenna Gain

The tag antenna gain, Gtag-ant, is the other important parameter for the reading distance Therange is largest in the direction of maximum gain which is fundamentally limited by the size,radiation patterns of the antenna, and the frequency of operation For a small dipole-likeomnidirectional antenna, the gain is about 0–2 dBi For some directional antennas such asthe patch antenna, the gain can be up to 6 dBi or more

3.3.2.4 Polarization Matching Coefficient

The polarization of the tag antenna must be matched to that of the reader antenna in order

to maximize the reading distance, which can be characterized by the polarization matching

because the orientation of the tag is random Using a linearly polarized tag antenna will result

−3 dB A circularly polarized tag antenna ispreferable for some specific applications because the signal can be increased by 3 dB

3.3.2.5 Power Transmission Coefficient

Referring to Figure 3.12, consider a tag antenna with a maximum effective aperture Ae-max(in square meters), situated in the field of the reader antenna with the power density S (wattsper square meter) It takes in power from the wave and delivers it to the termination, namelythe microchip with load impedance ZT Part of the power received by the tag antenna isdelivered to the termination while the rest of the power is reflected and re-radiated by theantenna The amount of the power delivered to the microchip can be quantified by using thepower transmission coefficient,  Let the power antenna received from the incident wave

be Ptag-ant, and the power delivered to the chip Ptag-chip Then

Ptag-chip= Ptag-ant (3.19)The power transmission coefficient, , is determined by the impedance matching between thetag antenna and the microchip Proper impedance matching between antenna and microchip

is of paramount importance in RFID since IC design and manufacturing are a big and costlyventure RFID tag antennas are normally designed for a specific microchip available on themarket Adding an external matching network with lumped elements is usually prohibitive

in RFID tags due to cost and fabrication issues To alleviate this situation, the tag antennacan be directly matched to the microchip which has complex impedance that varies with thefrequency and the input power supplied to the microchip

In the equivalent circuit shown in Figure 3.12(b), ZT= RT+ jXT is the complex chipimpedance and ZA= RA+ jXA is the complex antenna impedance The chip impedanceincludes the effects of chip package parasitics Both ZAand ZT are frequency-dependent Inaddition, the impedance ZT may vary with the power delivered to the chip

To describe the transmission of the power waves, we introduce a power wave reflectioncoefficient  [20]:

 = 0,  = 1.0, and the corresponding maximum transferred power is

When the antenna is shorted, the chip resistance RT= 0 and the chip reactance XT= −XA,

 will be unity and  zero Thus, there is no power delivered to the chip

It is convenient to relate the power transmission coefficient, , to another widely used

parameter, return loss (RL), for describing the impedance matching characteristics The return

loss is defined as:

It is convenient to obtain the return loss from simulation or/and measurement With thereturn loss, the corresponding reflection coefficient and the power transmission coefficientcan be easily calculated Table 3.4 shows the corresponding reflection coefficient and powertransmission coefficient for different return losses Figure 3.13 illustrates the relationshipbetween the power transmission coefficient and the return loss

Micro chip

Radar cross-section definition

The RCS is a measure of the amount of power scattered in a given direction when an object

is illuminated by an incident wave The IEEE defines RCS as 4 times the ratio of thepower per unit solid angle scattered in a specified direction to the power per unit area in

Figure 3.13 Transmission coefficient vs return loss.

a plane wave incident on the scatter from a specified direction More precisely, it is thelimit of that ratio as the distance from the scatter to the point where the scattered power ismeasured approaches infinity [19]:

for a large ship Due to the large dynamic range of the RCS, a logarithmic power scale ismost often used with the reference value of ref= 1 m2:

scattering, where the two directions are different In far-field RFID systems, backscattering

is used in the transmission of data from a tag to a reader

The RCS of an object is dependent on a range of parameters, such as size, shape, material,surface structure, polarization, and the operating wavelength The dependence of the RCS

on the operating wavelength generally divides objects into three categories:

• Rayleigh range The wavelength is much greater than the object dimensions, so there is

little variation in phase over the length of the body For objects smaller than around halfthe wavelength, the RCS exhibits a −4dependency and therefore the reflective properties

of objects smaller than 0.1 can be disregarded in practice

• Resonance range The wavelength is comparable with the object dimensions – typically

the object dimensions are taken to be between and 10 In this range, the electromagneticenergy shows a tendency to stay attached to the object’s surface to create surface wavesincluding traveling waves, creeping waves, and edge traveling waves Objects with sharpresonance, such as sharp edges, slits and points, may at certain wavelengths exhibitresonance set-up of RCS Under certain circumstances, this is particularly true for antennasthat are being irradiated at their resonant wavelength

• Optical range The wavelength is much smaller than the dimensions of object In this

case, only the geometry and position (angle of incidence of the electromagnetic wave) ofthe object influence the RCS

to radiate or receive RF energy and has a specific radiation pattern The principles of antennascattering modes are presented in Figure 3.14

The antenna RCS, , can conceptually be defined as

Although the concept of dividing the antenna RCS into two components is simple andeasily grasped, it should be noted that there is no formal definition of these scatteringmodes [19]

Figure 3.14 Antenna scattering: (a) antenna mode; (b) structural mode

3.3.2 .4 Polarization... variation in phase over the length of the body For objects smaller than around halfthe wavelength, the RCS exhibits a ? ?4< /sup>dependency and therefore the reflective properties

of objects... Preader-threshold, the maximumdistance for backscatter communication link can be obtained:

Rbackscatter= 4< /small>

2

of the